The mining of gypsum had begun in the GalloRoman period, first in open air mines and then underground, and continued until 1860. The gypsum was cut into blocks, baked, then ground and put into sacks. Sold as 'montmartarite, it was used for plaster, because of its resistance to fire and water. Between the 7th and 9th centuries, most of the sarcophagi found in ancient sites were made of .